Contemporary DRAM-based memory devices have become increasingly dense as the desire for more memory storage increases. Opening cell contacts in the devices has been a challenge in DRAM functionality. With each successive generation, the “open challenge” increases as design rules shrink.
By recessing an oxide (e.g., SiOx) of a recessed-access device (RAD) formed over active area (AA) silicon, more surface area for polysilicon deposition is exposed and better contact is achieved. However, traditional wet-fluorinated chemistries do not have a sufficiently high-level of oxide-over-nitride selectivity that is needed. Historically, the surface cleanliness of a silicon wafer could be achieved by using hydrofluoric (HF) acid (aqueous based) or buffered HF-solutions (e.g., buffered oxide-etch (BOE) chemistry types). These wet chemistries leave a hydrogen-bonded surface that is stable at room temperature, but only for a short period-of-time. During the short time period, a good silicon-to-silicon interface is possible. However, HF-based chemicals do not have the required selectivities with regard to silicon nitride isolation structures (for example, a silicon nitride layer that may surround a digit line in a memory device, such as DRAM memory). Selectivities of greater than 50:1 (oxide-to-nitride) are desired to keep the nitride layer intact. Otherwise, the electrical performance of the device is negatively impacted. Typically, oxide-to-nitride selectivity ratios for contemporary HF-based chemicals range from 10:1 to 40:1 depending on concentration, additives, and temperature. Therefore, alternative process methods are necessary for good cell contact Formation. Further, although various vapor-etch processes have been developed, contemporary processes are still incapable of oxide-to-nitride selectivity ratios of greater than 50:1 as is needed.